1. Field of the Invention
The present invention generally relates to a capacitance switching device for a keyboard, and particularly relates to a capacitance switching device of the membrane switch type used for a keyboard or the like.
2. Description of the Prior Art
Conventionally, there has been proposed a capacitance switching device for a keyboard (hereinafter referred to as "a capacitance keyboard switching device") in which the switching operation is performed by a change in electrostatic capacity between a movable electrode and a fixed electrode caused when a key on the keyboard is depressed.
As the capacitance keyboard switching device of the type as described above, there have been proposed capacitance keyboard switching devices of the membrane switch type as shown in FIGS. 1 and 2.
FIG. 1 shows a main part of a conventional capacitance keyboard switching device 10 in a keyboard. In the drawing, a fixed electrode 14 is provided on a back or lower surface of a first insulating film 12 which is the base side of the capacitance keyboard switching device 10, and a movable electrode 18 is provided on a back or lower surface of a second insulating film 16 disposed above the first insulating film 12, the fixed electrode 14 and the movable electrode 18 being disposed in opposition to each other in a switching opening portion 20 of a spacer 22 with a certain gap defined by the spacer 22. In the thus arranged capacitance keyboard switching device 10, when a key 24 (partially shown) is depressed, a depressing portion 28 of a movable electrode depressing member 26 is lowered to thereby depress the second insulating film 16 together with the movable electrode 18, so that the movable electrode 18 is made to come close to the fixed electrode 14 through the first insulating film 12 so as to increase an electrostatic capacity of a capacitor formed between the fixed electrode 14 and the movable electrode 18 to a predetermined value to thereby allow an externally applied AC signal to pass therethrough. The key 24 is constituted by a key top 24a, a cylindrical portion 24b, and an abutment portion 24c as shown in FIG. 2. The reference numeral 30 designates a bottom portion of a casing of the keyboard. It is a matter of course that the keyboard is provided with numbers of such capacitance keyboard switching devices while only one capacitance switching device is illustrated in the drawing. This applies in the following cases.
In the generally used conventional keyboard switching device 10 as described above, however, upon depression the movable electrode 18 cannot be kept flat at its circumferential part when depressed, so that the movable electrode 18 is bent by the elastic recovery force of the second insulating film 16 within the switching opening portion 20 of the spacer 22. Accordingly, the movable electrode 18 cannot completely contact with the first insulating film 12 at a portion of the latter over the fixed electrode 14, resulting in variations in value of the electrostatic capacity formed between the fixed electrode 14 and the movable electrode 18 when the movable electrode 18 is depressed by the depressing portion 28. Consequently, there has been a disadvantage that a proper switching operation cannot be performed.
In order to eliminate the foregoing disadvantage, there has been proposed an improved capacitance keyboard switching device as disclosed in Japanese Patent Unexamined Publication No. 117512/1985, in which a first fixed electrode is provided between a movable electrode and a second fixed electrode to thereby form a capacitor having a fixed capacitance between the first and the second fixed electrode and a switching operation is performed in the state where the movable electrode is caused to abut on the first fixed electrode to thereby prevent such a phenomenon that the electrostatic capacity is varied depending on the condition of the key depressing operation.
FIG. 2 shows such an improved capacitance keyboard switching device 10a. In the drawing, a first insulating film 36 is disposed in opposition to a second insulating film 40 with a predetermined gap therebetween defined by a spacer 22, a first fixed electrode 32 and a second fixed electrode 34 are respectively provided on the lower and upper surfaces of the first insulating film 36 in opposition to each other, and a movable electrode 38 is provided on the lower surface of the second insulating film 40 in opposition to the second fixed electrode 34. In the thus arranged capacitance keyboard switching device 10a, when depressed/released by a key 24 the depressing portion 28 comes down/up within an opening portion 20 of the spacer 22 to thereby cause the movable electrode 38 to directly contact with/separated from the second fixed electrode 34.
In such a conventional capacitance keyboard switching device 10a as described above, the fixed electrodes 32 and 34 are provided on the opposite surfaces of the first insulating film 36 to constitute a capacitor having a fixed capacity therebetween. Accordingly, it is necessary to form not only the fixed electrodes 32 and 34 but predetermined circuit patterns (not shown) on the opposite surfaces of the first insulating film 36, resulting in a problem that the structure of the keyboard switching device 10a is complicated to cause increase in manufacturing cost thereof.